In processing apparatuses for the processing of semiconductor substrates, shut-off valves are used for the control and switching of process gases. State of the art valves are diaphragm valves. The diaphragm is formed by a flexible membrane, which is, in a closing position of the valve, pressed against a seat around an opening, and thus closing the opening. In an open position the diaphragm is spaced from the seat allowing the passage of gas. An exemplary valve is described in U.S. Pat. No. 5,131,627 of Kolenc. A problem of shut-off valves is the dead volume: after shut-off of the valve process gas is still present in the dead volume downstream of the valve. This dead volume can be present in the valve device itself and/or in the tubing connected to the valve device. In a shut-off state of the valve, the process gas present in the dead volume can and will undesirably migrate further downstream through the tubing, giving rise to undesirable effects. In U.S. Pat. No. 5,542,452 of Carver, Jr et. al., a valve assembly is disclosed that exhibits zero dead volume. The valve comprises one inlet port and two outlet ports, all opening up at the membrane. In a closed position, the inlet port is closed-off by the membrane which is pressed against the valve seat, but the two outlets are in communication with each other. In an open position, the inlet is in communication with the outlets. After feeding a sample of a fluid across the valve seat from the inlet into the outlets, after closure of the valve the fluid sample can be transported away from the valve by passage of fluid between the two outlet ports, leaving no residues of the fluid sample behind within the valve device or in the tubing connected to the valve. Although dead volume effects are omitted in this way, this valve assembly still has the disadvantage that the quality of the shut-off fully depends on the quality of the seal of the membrane against the valve seat.
In case of Atomic Layer Deposition (ALD) wherein at least two mutually reactive reactants are fed to a reaction chamber in a sequence of alternating and repeating pulses, adequate separation of the different reactants is absolutely required, also after prolonged use of the system, resulting in possible wear. Additional problem is that many reactants used in ALD are at room temperature in liquid or solid state and have a very low vapour pressure. Therefore, the gas feed system needs to be heated, without any cold spots in the system that could result in condensation of the reactant. Consequently, the valve needs to operated at elevated temperature, which can be as high as 300° C. The performance of this type of valves under such conditions is relatively unknown.
It is the object of the present invention to provide a gas supply system that can shut-off a reactant without any dead volume effects and that can be operated to release well-separated pulses of a reactant while allowing a degradation of the seal between the valve member such as e.g. a membrane and the valve seat without deterioration of the separation of the pulses. It is a further objective of the present invention to provide a gas supply system and a valve assembly for low vapour pressure reactants, having no dead volume and capable of delivering repeated, well separated pulses of a reactant at an elevated operating temperature of the valve assembly to avoid condensation of the reactant.
It is a further object of the invention to provide a method for the switching of a reactant by a diaphragm valve that ensures the formation of repeated, well-separated pulses of the reactant, omitting dead-volume effects and allowing a degradation of the seal between the membrane and the valve seat without deterioration of the separation of the pulses.